a. Field of Invention
This invention pertains to a blade used on a doctor for a pulp or papermaking machine, and more particularly to a blade made of a fiber enforced composite material.
Pulp or papermaking machines, utilize machine rolls. Such machine rolls are used during various aspects of the process, for example, in the forming, pressing, drying or calendering sections. The operation of machine rolls requires a device to remove contaminants which form on the roll surface and/or to peel off a sheet or web from the rolls. A traditional method of achieving this is through the use of a mechanical device commonly referred to as a doctor or doctor blade. The failure to remove the contaminants or the sheet effectively can have a catastrophic effect on the quality of the product being produced.
The doctor blade is typically fastened to a structural beam which is adjustably supported across the papermaking machine on which a blade holder and a replaceable blade is provided. The doctor blade comes in direct contact with the roll surface so as to scrape off any contaminants from the roll surface including the whole pulp or paper web sheet or parts thereof.
b. Description of the Prior Art
There is a plurality of different doctor blade types having dimensions and materials commonly available in the industry, as well as different designs of blade holders. Laminated plastic doctor blades and blade holders such as type KF-35, KF-35A or PNEUFLEX blade holder are manufactured by Albany International Corporation, the assignee of the present invention. For obvious reasons the blade should be securely attached to the blade holder as a doctor without a blade will not scrape anything from the roll, and as aforesaid, this will have a catastrophic effect on the machine production. But even worst, the blade or a part thereof can come off and fall in the process where it will irreparably damage the pulp or paper machine clothing, and possibly the roll, because of direct and sudden contact with the blade holder.
The ultimate solution to prevent the aforesaid catastrophic situation would be to permanently fasten the blade to the holder or to make it as an integral part of the holder. But, doctor blades do wear with time. Depending on the application, they can last anywhere from a few hours to several months. Therefore, a doctor blade must be a replaceable item. The blade and holder design should allow for easy, fast and safe blade replacement so as to insure that neither the blade or part thereof, like the fastening devices for example, will come off and fall into the process.
A common design in the industry is to put along one edge of the blade, some types of rivets, or some other mechanical retainers that could be, for example, rivetted, glued, or press-fitted to the blade. The holder is then manufactured with a slot incorporating a step or a groove. The edge of the blade with the retainers can be slid into the groove through one end of the holder. Alternative designs are also available which allow a blade to be removed from the front of the holder, for the few applications where the access through the ends is limited. However, all these designs although widely used in the industry have a significant drawback as very often a retainer will come off the blade, and will either fall into the process, or will stay in the holder but become wedged into the blade slot, thus making the blade very difficult to slide in or out.
Another design used in the industry consist of making the blade with built-in retainers whereby there is no mechanically fastened part on the blade that can come off. One known way to do this is to machine the blade out of thicker material, leaving a narrow step along one edge that will retain the blade in the holder slot. This method is widely used to manufacture polyethylene doctor blades, where machining is fast and easy, and where thicker material is also required to add strength or to increase wear life. Theoretically, this method can be used to manufacture blades out of other popular materials, like metal or laminated plastic. However, the extended cost of the material and machining time combined with the high amount of tooling required, render this method simply undesirable. Moreover, it would not be suitable for the front removable blade design.
Another known way of making built-in retainers to the doctor blade is to stamp or punch pairs of short recesses along one edge of the blade at a certain spacing, to simulate the function of the rivets of the first design. A typical drawing of the industry standard is shown in FIGS. 1 and 2. However, this design has been used only to manufacture metallic doctor blades, such as bronze or stainless steel for example. It was believed that the mechanical properties of synthetic material used in the doctor blade industry, those of laminated glass fiber reinforced plastic, for example, did not allow this method to be used on plastic blades. All the laminated composite doctor blades known to be used on the pulp or paper machines today, are manufactured with add-on retainers that are either rivetted, glued, or press-fitted along one edge of the blade, a design with major disadvantages as described above. One such prior art rivetted composite doctor blade is shown in FIG. 3.